1. Field of the Invention
The invention relates to a projectile having a body of sintered metal powder.
2. Description of the Related Art
It is well known to protect the bore of the barrel of a firearm against the effect of the exposed projectile material (for instance lead), by providing the cartridge with an outer surface of, e.g., a copper alloy, thereby minimising coating problems resulting from direct contact between the core of the projectile and the wall of the bore and/or the bore rifling, while retaining the advantages relating to a high density of the projectile core (lead).
However, it has been realised in recent years that it may be fitting to avoid the use of lead in projectiles for rifle ammunition, revolver ammunition, etc., for environmental reasons, particularly when the ammunition shall be used on a practice range, and also when the ammunition is used for hunting purposes.
Consequently, there have been proposed in recent years projectiles that consist of a less environmentally harmful material, such as steel for instance. It is also known in this respect to produce the projectile body from a sintered metal powder. One problem with many such projectile materials, however, is that they can coat, scratch and/or damage the s of the bore in other ways. Because of this, it is known, e.g., from EP-A-0 626 557, to underdimension the actual projectile body in relation to the bore of the firearm and to produce the actual projectile body of sintered metal powder with a plastic covering whose thickness is preferably greater than the penetration depth of the bore rifling, so that the surface of the actual projectile body whose material (iron) is liable to scratch or score the barrel will not come into contact with the wall of the bore. This known solution requires, among other thigs, an additional manufacturing stage, i.e. a stage in which the sintered metal powder body is covered with plastic. In order to achieve good firing precision, i.e. a coherent hit pattern when using such projectiles, it is necessary for the plastic covering to be uniform and even on all projectiles. This is difficult to achieve with a plastic outer covering when applying known technology. Furthermore, if the plastic tends to coat the walls of the barrel, problems can occur with respect to precision.
It is also known to produce projectiles from copper or a copper alloy of full density. The advantages obtained in this regard that the material is considered environmentally friendly, has a relatively high density, a high toughness and a relatively low hardness, therewith enabling the copper projectile to be provided with a hollow tip, so that it can be used for hunting purposes and therewith expand upon impact.
One problem with the use of such copper projectiles or copper-alloy projectiles of fall density, however, is that the material has been found to be relatively hard despite everything else, and that the tolerance region for the engagement between projectiles of a given nominal calibre and the tolerance region for the bores of the weapons concerned can overlap each other to such an extent as to cause the gas pressure in the weapon to become unacceptably high and therewith promote the risk of the barrel bursting when using such copper projectiles. Furthermore, the precision of the weapon suffers when the gas pressure varies.
According to the present invention, this problem can be avoided without needing to reduce the diameter of the metallic part of the projectile and to coat the projectile with a plastic cover, by designing the projectile to include a body of sintered metal powder that faces towards the rifling in the bore of a firearm and therewith co-acts with the rifling, at least the outer surface of the body deformed by the rifling having a porosity in the range of 5-25%.
Further embodiments of the inventive projective are set forth in the accompanying dependent Claims.
In accordance with the invention, the sintered metal powder body of the projectile has a porosity such as to enable the body to be compressed radially with relatively slight resistance during passage of the projectile through the barrel of the weapon if the interference/tolerances require such compression, so that potentially dangerous radial interference between the bore of the barrel and the projectile can be reduced or avoided by the ensuing reduction or elimination of said porosity. Because this radial compaction of a projectile having such porosity can be effected with a lower force than if the projectile should comprise the same material but with fill density, it is possible to restrict friction of the projectile through the bore of the barrel and therewith also the gas pressure in the weapon.
In particular, when the body of the projectile is comprised of copper or a copper alloy, the sintered projectile body may have a porosity in the order of 10%. or generally between 5 and 25%. The projectile may conveniently comprise 90-95% by weight copper and 5-10% by weight zinc and/or tin, preferably about 95% by weight copper and 5% by weight zinc/tin. This minimises coating of the walls of the bore.
This porosity can be readily achieved by producing the projectile with the aid of a conventional sintering technique, wherewith a powder is compacted to form a body of desired shape and of desired dimensions and also with the desired porosity, and thereafter sintering the compacted powder body to desired mechanical strength, for instance to an extent such that the projectile material will have essentially full toughness. It is possible also in this way to obtain the advantage whereby the projectile will not normally splinter upon impact This enables the projectile to be formed to xe2x80x9ccushion-upxe2x80x9d upon impact, therewith enabling the projectile to be used for hunting purposes with a fully satisfactory result Furthermore, the projectiles can be easily collected from, e.g., the sand used in the construction of a practice firing range and reused when they are still substantially in one piece after striking the embankment on the firing range. It will be understood, however, that the invention also includes projectiles that splinter either completely or particulary upon impact with a target surface, which may be desirable in respect of, e.g., practice ammunition so that penetration of the projectile can be reduced. Particularly when the projectile shall be used as practice ammunition, a major volumetric part of the projectile may be produced from a metal powder, e.g., iron or steel, which results in lower costs.
Naturally, the inventive projectile can be further developed within the scope of the invention. For instance, the projectile may be given a core of some other material, for instance hardmetal, or a heavy material, such as tungsten, in order to give the projectile a relatively high density and/or certain determined ballistic properties.
In one embodiment of the invention, the projectile core can be formed from a material of full density. In another embodiment, the core can be formed from a powder different to its peripheral parts, wherewith the core powder may optionally be compacted, for instance in an axially exposed cavity in that part of the projectile that forms said outer parts. Similarly, the powder composition of the projectile may be varied along the longitudinal axis thereof.
It is important that the projectile has an outer layer of porous material, for instance tin or a tin alloy, that can be compacted radially essentially by eliminating or reducing said porosity as the projectile passes through the bore of the barrel, even in the case of troublesome tolerance overlap, so as to enable the projectile body to be made from a material which in a solid design would cause damage to the bore wall, for instance due to harness, or would be liable to promote a hazardous gas pressure in the weapon. In this regard, the material may also be chosen so as not to coat the lands of the rifling in the barrel, or only to a very small extent.
The inventive porosity of the projectile, at least in its radially outer parts, also enables the use of powder material which in addition to including a relatively large proportion of relatively soft basic material (copper, copper alloy) also contains a minor proportion of hard particles which can be pressed back into the porous, relatively soft material layer upon temporary contact with the wall of the bore.
When the projectile is formed from copper or alloys that have mechanical strength properties similar to those of copper and copper alloys, and densities in the same order of magnitude as that of copper, the porosity of the projectile may be in the region of 5-25%, preferably about 10%. On the one hand, the porosity of the projectile shall be sufficiently high to enable the projectile to be compacted radially with resistance, particularly its radially outer parts upon interference between the projectile and the wall of the barrel. On the other hand, the porosity of the projectile shall be so small that the decrease in density of the projectile due to the porosity will be as small as possible.
By way of example, it can be assumed that the projectile has a solid, completely rigid core with a porous outer layer that contacts the wall of the bore directly and engages the rifling of the barrel, in which case the thickness of the porous outer covering of the projectile will preferably be such that the radial compaction of the projectile required to avoid the generation of hazardous gas pressures can be achieved by elimination of said porosity.
The projectile can be produced from a unitary powder alloy that is compacted to form the projectile body, which is then sintered and preferably pressed to the desired calibration.
However, it may be desirable in some instances to form the projectile from two or more pre-manufactured complementary projectile parts which are joined and connected, preferably by pressing and suitably in conjunction with a calibration pressing process. These parts may have different compositions/alloying, such as to provide the projectile with a bore co-acting outer covering that has the aforementioned advantageous properties, namely a relatively high density, a radial compressiveness that is reduced by said porosity, a material selection that results in minimum coating of steel bore surfaces. The remaining parts of the projectile that do not thus co-act with the rifling of the bore, can be selected to impart other desired properties to the projectile. For instance, said other body parts may be comprised of tungsten or tungsten alloys when a high density is desired. When desiring a low cost, iron powder can be used, which also has a relatively high density. Bismuth and tin are also conceivable materials that result in a relatively low load on the environment.
In other respects, the manufacturing process is the same as that applied conventionally in the manufacture of metal objects by metal powder sintering techniques, wherewith the powder is conventionally compacted in a cold state to a desired body shape, whereafter said body is sintered at a temperature chosen to promote desired mechanical strength properties. If desired, the sintered body can be subjected to a calibration pressing process to give the body a more precise outer shape.
The inventive projectile is conveniently produced from a powder that consists manly of a copper alloy, for instance 95% by weight copper and 5% by weight zinc. Such an alloy has been found particularly favourable with respect to minimum coating of the bore surfaces. The powder may also include minor proportions of another powder that imparts other properties to the projectile, for instance a higher density. Even though such hard particles imply, per se, a scratching or scoring risk or the risk of elevated bore wear, the invention affords the advantage that the porosity of the body enables the hard particles to be pressed down more easily into the more resilient parts of the sintered body.
The inventive projectile may, for instance, be produced in calibre of up to 0.5 inch in diameter (heavy machine gun) or 20 mm in diameter (automatic canon), wherein the choice of material and porosity may also be used to limit the firing range of the weapon, which may be beneficial in conjunction with firing practice.